Few leakdown test questions

Last year my ski seemed like it was running lean. Each of the cylinders had very little wash. This is a 2000 slx, and I am running premix. The low and mid jets were stock, and I had very large (don't remember exactly which number) high jets.

I assumed I must have a problem with my carb, so last year I bought a perfect condition carb from someone on here. I swapped it in, and my ski seemed to run alot better (had better idle, I hit over 60 on gps).

Well, yesterday I decided to check the wash, the cen and pto were solid black, and it looked like there might be a hint of wash on the mag.

So, I'm thinking I might have an air leak somewhere. I already sprayed around the engine block with starting fluid to see if the idle changed, and it did not. I think I better do a leak down test on it.
-Can I do a leakdown test with the carbs on, and just plug each of the carb throats?
-I plan on putting air into one of the impulse lines, and having a gauge hooked up to the other; is this the proper way?

Thanks guys. Last summer when it was running lean, I replaced all the fuel lines with new. I also bypassed the water separator / selector and went directly from the tank to the carb.

This is the third carb I've had on my ski, and the second since the rebuild. The one I had on last summer I went through and cleaned out really good, and also installed the larger jets. Unless I have two bad carbs, I don't think that's the problem.

I know it should be jetted up for premix, (I had the last carb jetted up and it still ran lean), but the pistons are BLACK, with no sign of wash.

I'll look over the fuel lines again and make sure there is nothing loose. I guess the next logical step would be a leakdown test though.

Whats your compression numbers ? From what I understand if the compression is getting up there and the engine is getting close to detonating the wash will look lean no matting how much fuel you put in it. Try some higher octane fuel maybe.